A U.S. appeals court on Monday refused to lift a stay blocking major parts of Arizona's immigration law from taking effect and said the federal government is likely to be able to prove it is unconstitutional.

When Arizona passed a law in April allowing police to conduct roadside immigration checks, Mexican officials blasted the law as a prejudiced attack against its citizens in the state. That condemnation has spread throughout Latin America.

Arizona's immigration law faced tough scrutiny from a federal appeals panel Monday as the state's governor appeared in person to support the controversial provision on the day before the election in which she's seeking her first full term.

The nation's toughest immigration law has been in effect for three months. But after the federal courts prevented key portions from going into effect, it has failed to live up to both opponents' worst fears and supporters' greatest hopes.